The mechanisms involved in the biogenesis of mRNA are currently being investigated using purified vaccinia virus in vitro as a model system. A number of studies suggest that the biogenesis of mRNA may occur via a transcriptional precursor which is much larger than the final mature monocistronic message as has been proposed for eukaryotic mRNA synthesis in general. Such a mechanism involves the specific excision of a monocistronic RNA fragment from its putative precursor and the modification of that fragment at both the 5' and 3' termini with a methylated cap structure and polyadenylate chain respectively. Since many if not all of the structural and enzymatic components which are requisite for the biogenesis of mRNA via a processing mechanism appear to be present in purified vaccinia virions it appears as if vaccinia virus transcription in vitro may be a worthwhile system to explore to test the above hypothesis.